He passed away "a day or two ago".
This is about 4th hand, but it's the best you're going to get I'm afraid. He lived through Communist Poland, and was very privacy-conscious as a result. I have since met another priest from a former/Communist country, and guess what? He reminded me of Bp. Slupski almost to a T, even though his country was quite far (and different) from Poland. What they had in common? Being Catholic and knowing the Communist evils first-hand. I later learned, this priest was also consecrated bishop -- even when I knew him, although he was keeping it a secret at the time.
Anyhow, please pray for Bp. Slupski's soul. He was ordained in 1961 as a Redemptorist priest, and never said the Novus Ordo. He knew all the truth about the Conciliar religion, the Communists, the Freemasons, including the puppets who run the US government. He knew the truth about George Bush Sr. for example. He was a pioneer in the Trad movement, saying Mass in a half-dozen small independent chapels in the Midwest for several decades starting in the 1970's. He was always driving, always on the road. Sometimes with very little sleep.
I once accompanied him to one of his other chapels -- it was a singlewide trailer with a generator outside, but the inside was completely done up like a church. They had pews and everything. It was right on the border between Wisconsin and Minnesota -- it was quite a drive to get there, I recall.
It's true, he made a few mistakes when it came to "who to ordain" once he became a bishop -- but again, I ask for prayers for him. He did a lot of good for souls over the years, including my own, and many in my family. May God mercifully judge him, and go easy on him.